Network topologyFrom Wikipedia, the free encyclopediaJump to: navigation, search This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (December 2011) Network science Theory · History Graph · Complex network · Contagion Small-world · Scale-free · Community structure · Percolation · Evolution · Controllability · Topology · Graph drawing · Social capital · Link analysis · Optimization Reciprocity · Closure · Homophily Transitivity · Preferential attachment Balance · Network effect · Influence Types of Networks Information · Telecommunication Social · Biological · Neural · Semantic Random · Dependency · Flow Graphs Vertex · Edge · Component Directed · Multigraph · Bipartite Weighted · Hypergraph · Random
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Contents 1 Topology o 1.1 Point-to-point o 1.2 Bus o 1.3 Star o 1.4 Ring o 1.5 Mesh o 1.6 Tree o 1.7 Hybrid o 1.8 Daisy chain 2 Centralization 3 Decentralization 4 See also 5 References 6 External linksTopologyThere are two basic categories of network topologies: 1. Physical topologies 2. Logical topologiesThe shape of the cabling layout used to link devices is called the physical topology of thenetwork. This refers to the layout of cabling, the locations of nodes, and the interconnectionsbetween the nodes and the cabling. The physical topology of a network is determined by thecapabilities of the network access devices and media, the level of control or fault tolerancedesired, and the cost associated with cabling or telecommunications circuits.The logical topology, in contrast, is the way that the signals act on the network media, or the waythat the data passes through the network from one device to the next without regard to thephysical interconnection of the devices. A networks logical topology is not necessarily the sameas its physical topology. For example, the original twisted pair Ethernet using repeater hubs wasa logical bus topology with a physical star topology layout. Token Ring is a logical ringtopology, but is wired a physical star from the Media Access Unit.The logical classification of network topologies generally follows the same classifications asthose in the physical classifications of network topologies but describes the path that the datatakes between nodes being used as opposed to the actual physical connections between nodes.The logical topologies are generally determined by network protocols as opposed to beingdetermined by the physical layout of cables, wires, and network devices or by the flow of theelectrical signals, although in many cases the paths that the electrical signals take between nodes
may closely match the logical flow of data, hence the convention of using the terms logicaltopology and signal topology interchangeably.Logical topologies are often closely associated with Media Access Control methods andprotocols. Logical topologies are able to be dynamically reconfigured by special types ofequipment such as routers and switches.Diagram of different network topologies.The study of network topology recognizes eight basic topologies: Point-to-point Bus Star Ring or circular Mesh Tree Hybrid Daisy chainPoint-to-pointThe simplest topology is a permanent link between two endpoints. Switched point-to-pointtopologies are the basic model of conventional telephony. The value of a permanent point-to-point network is unimpeded communications between the two endpoints. The value of an on-demand point-to-point connection is proportional to the number of potential pairs of subscribers,and has been expressed as Metcalfes Law. Permanent (dedicated) Easiest to understand, of the variations of point-to-point topology, is a point-to-point communications channel that appears, to the user, to be permanently associated with the two endpoints. A childrens tin can telephone is one example of a physical dedicated channel. Within many switched telecommunications systems, it is possible to establish a permanent circuit. One example might be a telephone in the lobby of a public building, which is programmed to ring only the number of a telephone dispatcher. "Nailing down" a switched connection saves the cost of running a physical circuit between the two points.
The resources in such a connection can be released when no longer needed, for example, a television circuit from a parade route back to the studio. Switched: Using circuit-switching or packet-switching technologies, a point-to-point circuit can be set up dynamically, and dropped when no longer needed. This is the basic mode of conventional telephony.BusMain article: Bus networkBus network topology In local area networks where bus topology is used, each node is connected to a single cable. Each computer or server is connected to the single bus cable. A signal from the source travels in both directions to all machines connected on the bus cable until it finds the intended recipient. If the machine address does not match the intended address for the data, the machine ignores the data. Alternatively, if the data matches the machine address, the data is accepted. Since the bus topology consists of only one wire, it is rather inexpensive to implement when compared to other topologies. However, the low cost of implementing the technology is offset by the high cost of managing the network. Additionally, since only one cable is utilized, it can be the single point of failure. If the network cable is terminated on both ends and when without termination data transfer stop and when cable breaks, the entire network will be down. Linear bus The type of network topology in which all of the nodes of the network are connected to a common transmission medium which has exactly two endpoints (this is the bus, which is also commonly referred to as the backbone, or trunk) – all data that is transmitted between nodes in the network is transmitted over this common transmission medium and is able to be received by all nodes in the network simultaneously. Note: The two endpoints of the common transmission medium are normally terminated with a device called a terminator that exhibits the characteristic impedance of the transmission medium and which dissipates or absorbs the energy that remains in the signal to prevent the signal from being reflected or propagated back onto the transmission medium in the opposite direction, which would cause interference with and degradation of the signals on the transmission medium. Distributed bus The type of network topology in which all of the nodes of the network are connected to a common transmission medium which has more than two endpoints that are created by adding branches to the main section of the transmission medium – the physical
distributed bus topology functions in exactly the same fashion as the physical linear bus topology (i.e., all nodes share a common transmission medium). Notes: 1. All of the endpoints of the common transmission medium are normally terminated using 50 ohm resistor. 2. The linear bus topology is sometimes considered to be a special case of the distributed bus topology – i.e., a distributed bus with no branching segments. 3. The physical distributed bus topology is sometimes incorrectly referred to as a physical tree topology – however, although the physical distributed bus topology resembles the physical tree topology, it differs from the physical tree topology in that there is no central node to which any other nodes are connected, since this hierarchical functionality is replaced by the common bus.StarMain article: Star networkStar network topology In local area networks with a star topology, each network host is connected to a central hub with a point-to-point connection. In Star topology every node (computer workstation or any other peripheral) is connected to central node called hub or switch. The switch is the server and the peripherals are the clients. The network does not necessarily have to resemble a star to be classified as a star network, but all of the nodes on the network must be connected to one central device. All traffic that traverses the network passes through the central hub. The hub acts as a signal repeater. The star topology is considered the easiest topology to design and implement. An advantage of the star topology is the simplicity of adding additional nodes. The primary disadvantage of the star topology is that the hub represents a single point of failure. However, according to OBrien and Marakas, 2011, multiprocessor architecture has been commonly used as a solution to combat this disadvantage. Notes 1. A point-to-point link (described above) is sometimes categorized as a special instance of the physical star topology – therefore, the simplest type of network
that is based upon the physical star topology would consist of one node with a single point-to-point link to a second node, the choice of which node is the hub and which node is the spoke being arbitrary. 2. After the special case of the point-to-point link, as in note (1) above, the next simplest type of network that is based upon the physical star topology would consist of one central node – the hub – with two separate point-to-point links to two peripheral nodes – the spokes. 3. Although most networks that are based upon the physical star topology are commonly implemented using a special device such as a hub or switch as the central node (i.e., the hub of the star), it is also possible to implement a network that is based upon the physical star topology using a computer or even a simple common connection point as the hub or central node. 4. Star networks may also be described as either broadcast multi-access or nonbroadcast multi-access (NBMA), depending on whether the technology of the network either automatically propagates a signal at the hub to all spokes, or only addresses individual spokes with each communication. Extended star A type of network topology in which a network that is based upon the physical star topology has one or more repeaters between the central node (the hub of the star) and the peripheral or spoke nodes, the repeaters being used to extend the maximum transmission distance of the point-to-point links between the central node and the peripheral nodes beyond that which is supported by the transmitter power of the central node or beyond that which is supported by the standard upon which the physical layer of the physical star network is based. If the repeaters in a network that is based upon the physical extended star topology are replaced with hubs or switches, then a hybrid network topology is created that is referred to as a physical hierarchical star topology, although some texts make no distinction between the two topologies. Distributed Star A type of network topology that is composed of individual networks that are based upon the physical star topology connected in a linear fashion – i.e., daisy-chained – with no central or top level connection point (e.g., two or more stacked hubs, along with their associated star connected nodes or spokes).RingMain article: Ring network
Ring network topology A network topology that is set up in a circular fashion in which data travels around the ring in one direction and each device on the right acts as a repeater to keep the signal strong as it travels. Each device incorporates a receiver for the incoming signal and a transmitter to send the data on to the next device in the ring. The network is dependent on the ability of the signal to travel around the ring.Mesh It has been suggested that Fully connected network be merged into this article or section. (Discuss) Proposed since October 2011.Main article: Mesh networkingThe value of fully meshed networks is proportional to the exponent of the number of subscribers,assuming that communicating groups of any two endpoints, up to and including all the endpoints,is approximated by Reeds Law. Fully connectedFully connected mesh topologyThe number of connections in a full mesh = n(n - 1) / 2.
Note: The physical fully connected mesh topology is generally too costly and complex for practical networks, although the topology is used when there are only a small number of nodes to be interconnected (see Combinatorial explosion). Partially connectedPartially connected mesh topology The type of network topology in which some of the nodes of the network are connected to more than one other node in the network with a point-to-point link – this makes it possible to take advantage of some of the redundancy that is provided by a physical fully connected mesh topology without the expense and complexity required for a connection between every node in the network. Note: In networks that are based upon the partially connected mesh topology, the data that is transmitted between nodes in the network takes many shortest paths between nodes using a technology like Shortest Path Bridging, except in the case of a failure or break in one of the links, in which case the network uses the remaining alternative paths to the destination. This requires that the nodes of the network possess some type of algorithm to determine the correct path to use at any particular time.TreeTree network topology This section may be confusing or unclear to readers. Please help clarify the section; suggestions may be found on the talk page. (June 2011)The type of network topology in which a central root node (the top level of the hierarchy) isconnected to one or more other nodes that are one level lower in the hierarchy (i.e., the secondlevel) with a point-to-point link between each of the second level nodes and the top level centralroot node, while each of the second level nodes that are connected to the top level central root
node will also have one or more other nodes that are one level lower in the hierarchy (i.e., thethird level) connected to it, also with a point-to-point link, the top level central root node beingthe only node that has no other node above it in the hierarchy (The hierarchy of the tree issymmetrical.) Each node in the network having a specific fixed number, of nodes connected to itat the next lower level in the hierarchy, the number, being referred to as the branching factor ofthe hierarchical tree.This tree has individual peripheral nodes. 1. A network that is based upon the physical hierarchical topology must have at least three levels in the hierarchy of the tree, since a network with a central root node and only one hierarchical level below it would exhibit the physical topology of a star. 2. A network that is based upon the physical hierarchical topology and with a branching factor of 1 would be classified as a physical linear topology. 3. The branching factor, f, is independent of the total number of nodes in the network and, therefore, if the nodes in the network require ports for connection to other nodes the total number of ports per node may be kept low even though the total number of nodes is large – this makes the effect of the cost of adding ports to each node totally dependent upon the branching factor and may therefore be kept as low as required without any effect upon the total number of nodes that are possible. 4. The total number of point-to-point links in a network that is based upon the physical hierarchical topology will be one less than the total number of nodes in the network. 5. If the nodes in a network that is based upon the physical hierarchical topology are required to perform any processing upon the data that is transmitted between nodes in the network, the nodes that are at higher levels in the hierarchy will be required to perform more processing operations on behalf of other nodes than the nodes that are lower in the hierarchy. Such a type of network topology is very useful and highly recommended.definition : Tree topology is a combination of Bus and Star topology.HybridHybrid networks use a combination of any two or more topologies in such a way that theresulting network does not exhibit one of the standard topologies (e.g., bus, star, ring, etc.). Forexample, a tree network connected to a tree network is still a tree network topology. A hybridtopology is always produced when two different basic network topologies are connected. Twocommon examples for Hybrid network are: star ring network and star bus network A Star ring network consists of two or more star topologies connected using a multistation access unit (MAU) as a centralized hub. A Star Bus network consists of two or more star topologies connected using a bus trunk (the bus trunk serves as the networks backbone).
While grid and torus networks have found popularity in high-performance computingapplications, some systems have used genetic algorithms to design custom networks that havethe fewest possible hops in between different nodes. Some of the resulting layouts are nearlyincomprehensible, although they function quite well.A Snowflake topology is really a "Star of Stars" network, so it exhibits characteristics of a hybridnetwork topology but is not composed of two different basic network topologies beingconnected. Definition : Hybrid topology is a combination of Bus,Star and ring topology.Daisy chainExcept for star-based networks, the easiest way to add more computers into a network is bydaisy-chaining, or connecting each computer in series to the next. If a message is intended for acomputer partway down the line, each system bounces it along in sequence until it reaches thedestination. A daisy-chained network can take two basic forms: linear and ring. A linear topology puts a two-way link between one computer and the next. However, this was expensive in the early days of computing, since each computer (except for the ones at each end) required two receivers and two transmitters. By connecting the computers at each end, a ring topology can be formed. An advantage of the ring is that the number of transmitters and receivers can be cut in half, since a message will eventually loop all of the way around. When a node sends a message, the message is processed by each computer in the ring. If a computer is not the destination node, it will pass the message to the next node, until the message arrives at its destination. If the message is not accepted by any node on the network, it will travel around the entire ring and return to the sender. This potentially results in a doubling of travel time for data.CentralizationThe star topology reduces the probability of a network failure by connecting all of the peripheralnodes (computers, etc.) to a central node. When the physical star topology is applied to a logicalbus network such as Ethernet, this central node (traditionally a hub) rebroadcasts alltransmissions received from any peripheral node to all peripheral nodes on the network,sometimes including the originating node. All peripheral nodes may thus communicate with allothers by transmitting to, and receiving from, the central node only. The failure of a transmissionline linking any peripheral node to the central node will result in the isolation of that peripheralnode from all others, but the remaining peripheral nodes will be unaffected. However, thedisadvantage is that the failure of the central node will cause the failure of all of the peripheralnodes also,If the central node is passive, the originating node must be able to tolerate the reception of anecho of its own transmission, delayed by the two-way round trip transmission time (i.e. to andfrom the central node) plus any delay generated in the central node. An active star network hasan active central node that usually has the means to prevent echo-related problems.
A tree topology (a.k.a. hierarchical topology) can be viewed as a collection of star networksarranged in a hierarchy. This tree has individual peripheral nodes (e.g. leaves) which are requiredto transmit to and receive from one other node only and are not required to act as repeaters orregenerators. Unlike the star network, the functionality of the central node may be distributed.As in the conventional star network, individual nodes may thus still be isolated from the networkby a single-point failure of a transmission path to the node. If a link connecting a leaf fails, thatleaf is isolated; if a connection to a non-leaf node fails, an entire section of the network becomesisolated from the rest.To alleviate the amount of network traffic that comes from broadcasting all signals to all nodes,more advanced central nodes were developed that are able to keep track of the identities of thenodes that are connected to the network. These network switches will "learn" the layout of thenetwork by "listening" on each port during normal data transmission, examining the data packetsand recording the address/identifier of each connected node and which port it is connected to in alookup table held in memory. This lookup table then allows future transmissions to be forwardedto the intended destination only.DecentralizationIn a mesh topology (i.e., a partially connected mesh topology), there are at least two nodes withtwo or more paths between them to provide redundant paths to be used in case the link providingone of the paths fails. This decentralization is often used to advantage to compensate for thesingle-point-failure disadvantage that is present when using a single device as a central node(e.g., in star and tree networks). A special kind of mesh, limiting the number of hops betweentwo nodes, is a hypercube. The number of arbitrary forks in mesh networks makes them moredifficult to design and implement, but their decentralized nature makes them very useful. This issimilar in some ways to a grid network, where a linear or ring topology is used to connectsystems in multiple directions. A multi-dimensional ring has a toroidal topology, for instance.A fully connected network, complete topology or full mesh topology is a network topology inwhich there is a direct link between all pairs of nodes. In a fully connected network with n nodes,there are n(n-1)/2 direct links. Networks designed with this topology are usually very expensiveto set up, but provide a high degree of reliability due to the multiple paths for data that areprovided by the large number of redundant links between nodes. This topology is mostly seen inmilitary applications.See also Shared mesh Switched mesh Expander graph Scale-free network Network diagram Computer network
Relay network Cube-connected cycles Redundant topologies Tree structure Hierarchical clustering Internet topology Comparison of network diagram softwareReferences 1. ^ a b c d Groth, David; Toby Skandier (2005). Network+ Study Guide, Fourth Edition. Sybex, Inc.. ISBN 0-7821-4406-3. 2. ^ ATIS committee PRQC. "network topology". ATIS Telecom Glossary 2007. Alliance for Telecommunications Industry Solutions. Retrieved 2008-10-10. 3. ^ Proulx, S. R.; Promislow, D. E. L.; Phillips, P. C. (2005). "Network thinking in ecology and evolution". Trends in Ecology and Evolution 20 (6): 345–353. doi:10.1016/j.tree.2005.04.004. PMID 16701391. 4. ^ a b Inc, S., (2002). Networking Complete. Third Edition. San Francisco: Sybex 5. ^ Bicsi, B., (2002). Network Design Basics for Cabling Professionals. City: McGraw- Hill Professional 6. ^ Marakas, James A. OBrien, George M. (2011). Management information systems (Global ed., 10th ed. ed.). New York, NY: McGraw-Hill/Irwin. pp. 289. ISBN 978-0-07- 122109-2.External links
Advantages and Disadvantages of DifferentNetwork TopologiesA network topology refers to the way in which nodes in a network are connected to one another.The way in which they are connected defines how they communicate. Each kind of arrangementof network nodes has its own advantages and disadvantages. Here we tell you about the same.Network topologies describe the ways in which the elements of a network are connected. Theydescribe the physical and logical arrangement of network nodes. Let us look at the advantagesdifferent network topologies offer and get to know their shortfalls.Bus TopologyAdvantagesIt is easy to handle and implement.It is best suited for small networks.DisadvantagesThe cable length is limited. This limits the number of stations that can be connected.This network topology can perform well only for a limited number of nodes.Ring TopologyAdvantageThe data being transmitted between two nodes passes through all the intermediate nodes. Acentral server is not required for the management of this topology.DisadvantagesThe failure of a single node of the network can cause the entire network to fail.The movement or changes made to network nodes affects the performance of the entire network.Mesh TopologyAdvantageThe arrangement of the network nodes is such that it is possible to transmit data from one nodeto many other nodes at the same time.DisadvantageThe arrangement wherein every network node is connected to every other node of the network,many of the connections serve no major purpose. This leads to the redundancy of many of thenetwork connections.Star TopologyAdvantages
Due to its centralized nature, the topology offers simplicity of operation.It also achieves an isolation of each device in the network.DisadvantageThe network operation depends on the functioning of the central hub. Hence, the failure of thecentral hub leads to the failure of the entire network.What is a Topology?The physical topology of a network refers to the configuration of cables, computers, and otherperipherals. Physical topology should not be confused with logical topology which is the methodused to pass information between workstations. Logical topology was discussed in the Protocolchapter.Main Types of Physical TopologiesThe following sections discuss the physical topologies used in networks and other related topics. Linear Bus Star Tree (Expanded Star) Considerations When Choosing a Topology Summary ChartLinear BusA linear bus topology consists of a main run of cable with a terminator at each end (See fig. 1).All nodes (file server, workstations, and peripherals) are connected to the linear cable.Fig. 1. Linear Bus topology
Advantages of a Linear Bus Topology Easy to connect a computer or peripheral to a linear bus. Requires less cable length than a star topology.Disadvantages of a Linear Bus Topology Entire network shuts down if there is a break in the main cable. Terminators are required at both ends of the backbone cable. Difficult to identify the problem if the entire network shuts down. Not meant to be used as a stand-alone solution in a large building.StarA star topology is designed with each node (file server, workstations, and peripherals) connecteddirectly to a central network hub, switch, or concentrator (See fig. 2).Data on a star network passes through the hub, switch, or concentrator before continuing to itsdestination. The hub, switch, or concentrator manages and controls all functions of the network.It also acts as a repeater for the data flow. This configuration is common with twisted pair cable;however, it can also be used with coaxial cable or fiber optic cable.Fig. 2. Star topologyAdvantages of a Star Topology Easy to install and wire. No disruptions to the network when connecting or removing devices. Easy to detect faults and to remove parts.
Disadvantages of a Star Topology Requires more cable length than a linear topology. If the hub, switch, or concentrator fails, nodes attached are disabled. More expensive than linear bus topologies because of the cost of the hubs, etc.Tree or Expanded StarA tree topology combines characteristics of linear bus and star topologies. It consists of groupsof star-configured workstations connected to a linear bus backbone cable (See fig. 3). Treetopologies allow for the expansion of an existing network, and enable schools to configure anetwork to meet their needs.Fig. 3. Tree topologyAdvantages of a Tree Topology Point-to-point wiring for individual segments. Supported by several hardware and software venders.Disadvantages of a Tree Topology Overall length of each segment is limited by the type of cabling used. If the backbone line breaks, the entire segment goes down.
More difficult to configure and wire than other topologies.5-4-3 RuleA consideration in setting up a tree topology using Ethernet protocol is the 5-4-3 rule. One aspectof the Ethernet protocol requires that a signal sent out on the network cable reach every part ofthe network within a specified length of time. Each concentrator or repeater that a signal goesthrough adds a small amount of time. This leads to the rule that between any two nodes on thenetwork there can only be a maximum of 5 segments, connected through 4repeaters/concentrators. In addition, only 3 of the segments may be populated (trunk) segments ifthey are made of coaxial cable. A populated segment is one that has one or more nodes attachedto it . In Figure 4, the 5-4-3 rule is adhered to. The furthest two nodes on the network have 4segments and 3 repeaters/concentrators between them.NOTE: This rule does not apply to other network protocols or Ethernet networks where all fiberoptic cabling or a combination of a fiber backbone with UTP cabling is used. If there is acombination of fiber optic backbone and UTP cabling, the rule would translate to a 7-6-5rule.The speed of networking switches is vastly improved over older technologies, and whileevery effort should be made to limit network segment traversal, efficient switching can allowmuch larger numbers of segments to be traversed with little or no impact to the network.Considerations When Choosing a Topology Money. A linear bus network may be the least expensive way to install a network; you do not have to purchase concentrators. Length of cable needed. The linear bus network uses shorter lengths of cable. Future growth. With a star topology, expanding a network is easily done by adding another concentrator. Cable type. The most common cable in schools is unshielded twisted pair, which is most often used with star topologies.Summary ChartPhysical Topology Common Cable Common Protocol Twisted Pair Linear Bus Coaxial Ethernet Fiber Twisted Pair Star Ethernet Fiber Twisted Pair Tree Coaxial Ethernet Fiber